Experimentally supported consideration of operating point dependent soil properties in coupled dynamics of offshore wind turbines

Abstract

The consideration of soil properties is necessary to predict the time domain dynamic behavior of offshore wind turbines. Accurate soil-structure interaction models are in essence very expensive in terms of computing time and therefore, not directly applicable to transient calculations of wind energy converters. In this work, the incorporation of dynamic soil properties is addressed. The basic model, previously developed by the authors, is based on a linearized approach using stiffness and mass matrices representing the soil-structure interaction. This approach already leads to significant reductions of the eigenfrequencies compared to clamped boundary conditions which are still commonly used. Here, the basic approach is enhanced by two aspects. Firstly, different numerical soil models, based on nonlinear springs, to calculate the matrices are compared to experimental results for embedded piles at conditions similar to the North Sea. Comparisons of numerically and experimentally determined eigenfrequencies of the piles show that nonlinear spring models are only suitable for dynamic analyses to a limited extent. Secondly, a piecewise defined response surface, which enables a linearization of the nonlinear soil behavior at different approximated operating points, is introduced. This approximation proves to be sufficiently accurate in the current setting. By analyzing two full offshore wind turbine examples in time domain, a monopile substructure and a jacket substructure anchored by piles, further shifts of the eigenfrequencies, being caused by the load-dependent mechanical properties of the soil, are determined by considering the operating point.